23 November Internal Resistance Questions

The topic of Internal Resistance builds on and uses many of the concepts and equations we've learned earlier – it’s a good summative topic.

All you really need for internal resistance questions are a good understanding of:

• Ohm's law V = IR
• The fact that potential differences add up in series
• To remember that p.d.s or voltages are measuring the energy of the charge, in joules per coulomb
• In addition, that the 'lost volts' is the p.d. 'across' the internal resistance.  Lost volts = Ir where r is the internal resistance
• It might also be useful in some questions to know that Power = VI = I2R
• Power dissipated in the cell or other source = I2r
• Also, power from the source is e.m.f. x current = Iε

By doing questions on internal resistance you will be revisiting these topics – this will consolidate your understanding and recall.

Remember that the internal resistance equation is in the formula book:

 V_load = ε – Ir

Also, remember that V_load = IR where R is the load resistance, Ir = the lost volts and ε is the e.m.f. of the source

Questions from the Internal Resistance booklet:  Internal resistance of power supplies.


Exam Questions Q 1.  Remember that power = VI =  I2R


Exam Questions Q 2.  Involves some algebra!


Exam Questions Q 3.  This is quite a challenging question requiring you to think about uncertainties carefully.